This invention relates to a contactless label for antitheft or identification purposes which comprises an electric circuit, to an electric circuit for such a label and to a deactivation and/or coding system comprising such labels. This invention relates in particular to the deactivation or coding of electromagnetic antitheft labels which are used only once and so are not removed during checkout at a checkout counter, as well as to the coding or programming of identification labels for the purpose of contactless identification of humans, animals or goods.
The operation of known electromagnetic antitheft systems with single-use labels is based on two principles. One system utilizes an electromagnetic radiofrequency (RF) field of a frequency of approximately 8 MHz and works with labels or tags which contain a tuned circuit consisting of a coil and a capacitor. The other system utilizes an electromagnetic RF field of a frequency of approximately 4 kHz and works with labels containing a strip of amorphous metal having specific magnetic properties.
The present invention relates to the first type of antitheft system, where the coil and the capacitor in the tags are typically formed by aluminum foil provided on two sides of a carrier material of polyethylene (PE) or polypropylene (PP). The formation of this coil and capacitor is effected by means of an etching process, with prior arrangement of a mask on the portions which are not be etched away. An example of a label based on a principle as described above is the label according to applicant's European patent no. 0,280,361.
The tags for the above-mentioned antitheft systems are mostly fitted in the store where the goods are sold. When the goods are being checked out at a checkout counter, the tags are deactivated, so that no alarm is produced when the goods leave the store via the detection systems arranged at the exit. For deactivation of the tags, mostly a system is used which effects an electric breakdown at a weak spot priorly provided in the capacitor, which causes a short circuit rendering the tag inactive in the antitheft system.
The provision of this weak spot in the capacitor can be effected in the manner as indicated in U.S. Pat. No. 4,567,473 (Checkpoint), where a weak spot is provided through forcing. It can also be effected, for instance, in the manner according to U.S. Pat. No. 5,187,466 (Fritz Pichl), where by means of electric contacts a break is provided during the production process, which break is then removed again by passing a current through the break formed, so that a weak spot remains.
A further alternative is the method according to European patent application 0,472,073, also of Fritz Pichl, whereby through a chemical reaction conduction is effected to generate a short circuit.
All of these methods have the disadvantage that the short circuit formed in the capacitor during deactivation is highly vulnerable and sensitive to mechanical movements because the short circuit formed consists of an extremely thin layer of condensed aluminum. Accordingly, it happens regularly that this short circuit disappears spontaneously so that the label becomes active again and will set off an alarm, for instance in a next store which is being visited. This, of course, leads to undesirable situations.